[clang] Invalid rejection of enum arithmetic

[davidstone]

The following valid translation unit

enum e1 {
	value = 0
};

enum e2 {
	a = 0,
	b = e1::value,
	c = a + b
};

is incorrectly rejected by clang when compiled in C++26 mode with

<source>:8:8: error: invalid arithmetic between different enumeration types ('e2' and 'e1')
    8 |         c = a + b
      |             ~ ^ ~
1 error generated.
Compiler returned: 1

See it live: https://godbolt.org/z/v4sG9YqMb

In earlier versions of C++, it incorrectly warns saying the conversion is deprecated.

This was accepted in clang 18.1.0, but is failing on current trunk (15e9478).

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: David Stone (davidstone)

The following valid translation unit

enum e1 {
	value = 0
};

enum e2 {
	a = 0,
	b = e1::value,
	c = a + b
};

is incorrectly rejected by clang when compiled in C++26 mode with

<source>:8:8: error: invalid arithmetic between different enumeration types ('e2' and 'e1')
    8 |         c = a + b
      |             ~ ^ ~
1 error generated.
Compiler returned: 1

See it live: https://godbolt.org/z/v4sG9YqMb

In earlier versions of C++, it incorrectly warns saying the conversion is deprecated.

This was accepted in clang 18.1.0, but is failing on current trunk (15e9478).

[Sirraide]

This seems to be a bug, yeah. The type of a within e2 should be int; b is initialised by an expression of unscoped enumeration type, for which the standard specifies that it should have the underlying type of that enumeration [dcl.enum].

[shafik]

Related: #84068

[Sirraide]

Related: #84068

Ah, I somehow was under the impression that that had already been merged.